Quantum dots (QDs) are of interest in a number of fields, including electronics, photovoltaics, computing, imaging, catalysis, and medicine. Quantum dots are a form of semiconductor nanoparticles having a small size on the order of about 1 to about 100 nm, or from about 1 to about 10 nm, and which may have optical, electronic, or magnetic properties which vary from those of bulk materials. In particular, many of these properties may vary with the size of the individual dots. That is, one or more material properties can be altered or changed by merely a change in size of the dot through the angstrom and nanometer scales. For example, in general conductivity of metals typically goes down as temperature goes up. The rate of change of conductivity with temperature may be altered by size changes in the nanometer scale. As a result, tight control over size is an important factor in the manufacture of quantum dots. These properties may vary with the size of the particles either in a linear or continuous proportion with size changes, or in a quantized fashion, that is the value associated with a given property may change in a step-wise manner at particular sizes of the particle, but will be relatively constant in-between those steps.
Other factors that can affect the quality or commercial manufacture of quantum dots include purity and stoichiometry. For example, the lack of control over purity, such as by the presence of contaminants may interfere with the formation of the desired crystalline structure or result in broadening of the electronic and optical characteristics of the particles.
Stoichiometry of quantum dots is an important consideration. The particular stoichiometry may control the ability of particles to form at all, as well as characteristics (electrical, optical, magnetic, etc.) of the quantum dot. Some quantum dots may have two or more components which are each present in substantially comparable concentrations, e.g. 1:1, 2:1, 3:1, 4:1, 5:1, 10:1. Other quantum dots may have one or more dopants which are present in much smaller quantities, such as 100:1, 500:1, 1000:1. Quantum dots may also have both one or more stoichiometric components and one or more dopants.
Precise control of particle size may also critical to many applications. The electrical and optical characteristics of any given particle may depend on its size, whether in a quantized or proportional relationship. In addition to the size of individual particles, the distribution of size within a population of the same quantum dot may also been important. In many applications, a narrow size distribution will be desired so that characteristics that correspond to size will also be relatively narrow, e.g., narrow absorbance and emission bands. Size effects may be proportional within certain size domains and may be quantized within certain other size domains.
Existing synthetic methods for forming quantum dots may largely be grouped as physical and chemical methods. Physical methods include inert gas condensation, arc discharge, ion sputtering, laser ablation, and pyrolysis. Chemical methods may be grouped under various categories, such as by solvent used (aqueous or organic) or conditions of the reaction (e.g., solvothermal reactions carried out above the normal boiling point of the solvent by applying high pressure, or arrested precipitation reactions at high pH).
Conventional synthetic methods are plagued with multiple problems. For example, chemical methods may utilize hazardous or expensive solvent precursors. In addition, disposal of waste solvents can be expensive. Further, efficient conversion of precursor material plagues both synthetic method routes. That is, typically a large amount of waste is generated in processing the precursor materials in comparison to the amount of quantum dots manufactured.
Another measure of efficiency relates to control over particle size. Where particle size is widely distributed, it may be necessary to separate the desired product particles and discard particles outside the appropriate tolerance. This results in a further reduction is efficiency.